No, that is not what JTWROS means.  Code Section 2040 means that the ENTIRE property would be included in his taxable estate (not probate estate, which is irrelevant  here).  That means that the only number needed for cost basis is FMV at date of death.  This is a simple question but the path to its answer here has been littered with red herrings, and @Mary M 's client appears still at risk of paying too much tax.